http://www.ck12.org Chapter 25. Organic Chemistry
In theFigure25.6, the two ends of the chain and each of the intervening corners represents a carbon atom. The six
carbon atoms are connected in a linear chain by single bonds. Unless indicated otherwise, we also assume that each
carbon makes four total bonds, and any bonds that are not explicitly drawn are connected to hydrogen atoms. The
internal carbon atoms above each make single bonds to two other carbons, leaving two bonds not shown. Thus, each
of these carbon atoms is connected to two hydrogens. The carbons on the ends of the chain only have one covalent
bond drawn in, so they must each be bonded to three hydrogen atoms. Compare these two representations of hexane,
keeping in mind that both are conveying the same information.
Locating Functional Groups
Most organic compounds are not simple hydrocarbons; they have functional groups that provide additional reactivity
pathways. To indicate the location of a functional group within the name of an organic molecule, the hydrocarbon
backbone is generally numbered. For example, the hexane molecule (seeFigure25.5) could serve as a parent chain.
It has six carbons in it, which can be numbered C-1, C-2, and so on. As long as there is nothing else attached to
the chain, it does not matter where we start counting. There is no way to designate which carbon is C-1 and which
carbon is C-6. However, once a substituent is added to the chain, we can then indicate a start and an end to the
molecule.
Now, let’s introduce a functional group by replacing one of the hydrogen atoms in hexane with a chlorine atom:
FIGURE 25.7
2-ChlorohexaneTo indicate that this molecule has a chlorine atom attached to the hydrocarbon backbone, we could name this
compoundchlorohexane. However, that name would not be enough information to uniquely identify this molecule,
since the chlorine could be attached to any of the carbon atoms. To indicate the location of this substituent, we
number the chain, starting with the end that will place the functional group on the carbon atom with the lowest
number. Depending on which end is C-1, the compound in theFigure25.7 could be called either 2-chlorohexane
or 5-chlorohexane. According to our rule about giving functional groups the lowest possible numbers, this molecule
would be called 2-chlorohexane.
Other alkanes with a single halogen atom can be named using a similar strategy, except chloro would be replaced by
fluoro, bromo, or iodo, depending on the identity of the halogen.
The location of double and triple bonds must also be indicated with numbers. For example, consider the following
two molecules: CH 3 CH 2 CH=CHCH 3 and CH 2 =CHCH 2 CH 2 CH 3. Both of these have one double bond. A simple
5-carbon alkane (no double bonds) would be called pentane, so adding in one double bond changes the name of the
structure to pentene, since it is an alkene. However, the location of the double bond affects the physical and chemical
properties of the compound.
In order to distinguish between the two molecules above, we again number the carbon chain, starting from the
end that will give the functional group (the alkene) the lowest number. For CH 3 CH 2 CH=CHCH 3 , we would start
counting on the right end. The double bond is between carbon atoms 2 and 3, so this molecule would be named
pent-2-ene, where the lower of the two numbers is used. For CH 2 =CHCH 2 CH 2 CH 3 , we would start counting on the
left end. The double bond is between carbon atoms 1 and 2, so this molecule would be named pent-1-ene.